How to remove someone from the blocked list on your iPhone or iPad:
You can totally block callers and errant iMessagers that make you angry or spam you with BS. But if you accidentally block someone you didn't want to or you blocked a number before you realized who it was, you can unblock them just as easily.

Here's how!
How to unblock a phone number or email address on iPhone and iPad
Launch the Settings app from your Home screen.
Tap on Phone, Messages, or FaceTime. Any one of these three sections will work.

Tap Edit in the upper right corner of your screen.
Tap the minus button (red circle) next to the number or email address you'd like to unblock.

Tap Unblock.

You can also swipe left on the number or email address you'd like to unblock and tap Unblock that way.
See Video:

That's all there is to it. The nice thing about the blocked list in iOS is that you can have only a certain email address or phone number for a person blocked, and you can remove only certain ones if you'd like. It makes the feature a little more versatile than other blocking features that are all or nothing.